1. Field of the Invention
This invention relates generally to the field of power dividers or combiners in RF or microwave frequencies and, in particular, to an N-way power divider/combiner possessing switching capabilities for selectively controlling operative modes of its constituent dividing/combining channels.
2. Description of the Prior Art
RF or microwave power divider/combiners are used in the electronics industry to either divide or combine RF or microwave signals. When operating as a power divider, one input signal is divided into a plurality of output signals, each retaining the same signal characteristics but having a lower power level than the input signal. As a power combiner, a plurality of input signals is combined into a single output signal, with the output signal having the signal characteristics of the sum of the plurality of input signals. Thus, a divider/combiner can operate as either a power divider or a power combiner, depending on the direction of the signals.
In a typical application in RF or microwave systems, a divider/combiner used as a power divider divides an input signal into a plurality of equi-phase, equi-amplitude signal outputs for amplification by power amplifiers and, when used as a power combiner, combines the outputs of several power amplifiers together to attain a useful power output. In such systems, it is usually desired that if one of the individual amplifiers fails, the system can continue to operate with a minimum reduction in the system output power and with a minimum degradation in the signal characteristics. Therefore, it is advantageous that the power divider/combiner exhibits a low overall power loss, is symmetrical in configuration to avoid phase and amplitude imbalances, and provides sufficient isolation and impedance matching between its output ports (or input ports). Furthermore, the characteristics of low loss, electrical symmetry, isolation and impedance matching should be maintained in a normal operating mode as well as in the presence of a failed amplifier in order to prevent the failure of a single amplifier from seriously degrading the performance of the divider/combiner circuitry or the remaining amplifiers.
A low-loss power divider/combiner which utilizes a circular or radial symmetry and provides isolation and impedance matching is described in an article entitled "An N-Way Hybrid Power Divider" by E. J. Wilkinson, IRE Transactions on Microwave Theory and Techniques, vol. MTT-13, pp.116-118, January 1960. FIG. 1 shows a schematic representation of a conventional Wilkinson hybrid power divider. Although the Wilkinson divider in FIG. 1 is depicted in a four-way configuration, its operation will be described, for the purpose of generality, in terms of an N-way device.
In an N-way Wilkinson hybrid divider 10, as shown in FIG. 1, an RF signal fed into an input port 12 with an input impedance Z.sub.0 is divided into N equi-phase, equi-amplitude signals by way of N transmission lines 14 each having an electrical length of one-quarter wavelength (.lambda./4) at the operating frequency. The output end of each transmission line 14 is further coupled to each of N output ports 16 having an output impedance Z.sub.0. Isolation between the N output ports 16 is accomplished by means of N resistors 18 each connected between the output end of each transmission line 14 and a common node 20, and having a resistance R.sub.0. When the characteristic impedance Z of the N transmission lines 14 is set to .sqroot.NZ.sub.0 and R.sub.0 is equal to Z.sub.0, the output ports 16 are optimally matched and isolated. As a result, no power is dissipated in the resistors 18 and 1/N of the input power is delivered to each of the N output ports 16. Likewise, when used as a power combiner, the Wilkinson arrangement can combine N RF input signals of power P.sub.in applied to the previously called N output ports 16 into an output signal of power N.times.P.sub.in if the input signals are in phase and of equal amplitude. However, if the input signals are not equi-phase equi-amplitude, a substantial portion of power is dissipated by the resistors 18 and the power delivered to the previously called input port 12 will be reduced accordingly.
Several modified versions of the Wilkinson divider/combiners are known in the prior art. U.S. Pat. No. 5,410,281, issued to Blum, discloses a high power combiner/divider which provides effective cooling of the isolation resistors by locating the resistors remote from the body of the device using extended transmission lines. Additional Wilkinson-type divider/combiners, including planar radial hybrids and "fork" hybrids, are described in an article, "Planar Electrically Symmetric n-way Hybrid Power Dividers/Combiners" by Adel A. M. Saleh, IEEE Transactions on Microwave Theory and Techniques, vol. MTT-28, pp. 555-563, June 1980. FIG. 2 and FIG. 3 show the schematic representation of a prior art radial hybrid and a fork hybrid, respectively. Several variations of the planar radial/fork hybrids of the Wilkinson-type are also disclosed in U.S. Pat. No. 4,129,839 to Galani, U.S. Pat. No. 5,021,755 to Gustafson, and U.S. Pat. No. 5,455,546 to Frederick et al.
The conventional hybrid power divider/combiners, such as the Wilkinson divider, suffer from several shortcomings. When one of the N signal-processing channels coupled to the divider outputs experiences a failure or malfunctions, such as in an amplifier failure, the conventional hybrid power divider continues to distribute the power to the disabled channel and 1/N of the power is lost through the disabled channel. In a multi-channel divider/combiner network, the failure of an operating channel also introduces a mismatch between output or input impedances of the divider/combiner, which degrades the overall performance characteristics of the network. Another significant disadvantage of the conventional Wilkinson hybrid is that when used as a combiner, if one or more of the power amplifiers in the combiner network fail, any imbalance in the output voltages of the power amplifiers induces voltages across the isolation resistors, further reducing the combiner output power as the power from the remaining operational amplifiers is divided between the isolation resistors and the combiner circuit. For a network combining N power amplifiers, the decrease in power (P.sub.d) resulting from disabling M of the N power amplifiers is given by the equation, P.sub.d (dB)=10 log((N-M)/N).sup.2. Thus, when one of the amplifiers fails in a two-amplifier combining network, the output power of the conventional hybrid combiner drops not by 3 dB, but by 6 dB. Such additional power loss is unacceptable in many applications and results in excessive heat dissipation in isolation resistors, limiting the power handling capabilities of the combiner, especially in high power situations. Although Blum's patent attempts to ameliorate some of the problems arising from the excessive heat dissipation in the resistors, it does not solve any of the more fundamental shortcomings herein described.
The conventional Wilkinson hybrids also present serious packaging problems. In practice, the device is difficult to realize in a planar structure when N is greater than 2 as the resistors need to be connected to a floating common node. For example, in microstrip versions, such Wilkinson hybrids are subject to signal crossover and cross coupling, which adversely affect the isolation characteristics of the circuit. Although the radial or fork hybrids of Galani, Gustafson, and Frederick attempt to solve some of these problems, they still suffer from the impedance mismatching and the inefficient power distribution/combining in a failure mode. Furthermore, the conventional Wilkinson hybrids do not provide enough room for internally accommodating the means for individually controlling the dividing/combining channels, such as RF switches, due to the structural limitations in packaging the resistors and the physical proximity of the transmission lines and the resistors.
What is needed is an improved power divider/combiner which can be operated in a normal operating mode as well as in the presence of a failed channel, which can maintain the impedance lo matching of the circuitry and achieve efficient power combining and distribution even in the failure mode, and which can be easily implemented without the structural and physical limitations of the conventional Wilkinson divider/combiners.